How To Make HOT Compost

on May 9, 2014

Hot composting is a great way to process bulk organic matter and process (get rid of) any pathogens (bad bugs) and unwanted plants (weeds). You can spend your whole life studying and practicing compost and still not know everything – it’s such a deep and intricate science. This brief article is merely here to help point you in the right direction in larger scale composting, keep researching and practicing and you’ll just keep learning!

What’s the difference between hot and cold composting?

Hot composting involves building a compost pile (in one go) which is at least one cubic metre in size, any less than this and it wont be able to generate the heat required to break down the organic matter and kill off pathogens (bad bugs) in the desired time frame. Hot composting will easily get up to over 100 degrees if you let it, however the desired temperature is 60/65 degrees. At this level it’s killing off the pathogens (the bad bugs) but not the desired biology, once it gets hotter than this the good biology is also getting killed – not good.

Nifty thermometers built for the job can help keep an eye on the compost pile’s temperature. Image from here

Cold compost is… you guessed it, cold. It’s usually done on a smaller scale in standard compost bins and is more popular with people with tiny garden spaces. You add to slow compost systems gradually, a small bucket of food scraps and straw every day or so.

There are two things that all composting methods have in common, the first thing is the ingredients. There are 4 universal inputs: carbon, nitrogen, water and air. Carbon ingredients is anything that’s dry and brown (think dead), such as straw, hay, brown leaves, shredded office paper, ripped/scrunched newspaper, cardboard – you get the idea. Nitrogen is anything that’s really fresh including animal manures (horse, cow, chicken, sheep, rabbit etc (leave out cat poo due to the risk of totoxoplasmosis)), green lawn clippings, food scraps and green waste. There are some things I don’t ever put in the compost as it feels wrong, these include invasive grass species (twitch and kikuyu and any seed heads from plants I really don’t wont in my garden). I’ll dry and burn these, give them to the cickens or drown them in a bucket of water instead of putting them in the compost. But that’s just me.

The second thing all compost methods have in common is that these ingredients are layered. Just like a lasagna, the carbon and nitrogen materials are layered, alternating between the two until you’ve reached at least one cubic metre as seen below.

The layer of twigs at the bottom helps increase airflow and drainage from the pile. Once that’s down you can get started with your carbon and nitrogen layers.

This photo is actually from a small compost bin, however you apply this same technique on a large scale when building a hot compost. You may notice there’s some wire mesh on the bottom – this is an add on to prevent rodents from moving in, a handy tip if you have unwanted furry animals raiding your compost bin.

What ratio of carbon and nitrogen materials should you use?

Most compost books you read will say stick to a carbon/nitrogen ration of 25:1. However it really depends on the materials you have on hand, you’ll find that very rarely will any of them actually be 100% carbon or nitrogen, they’ll always have some of the other in them – if that makes sense. Personally, I’ve found using a ratio of 50:50 works well, sometimes a little less nitrogen and more carbon when I have particularly rich nitrogen ingredients – just to make sure it’s balanced.

I like making compost with friends, time goes quickly, you learn new things (about each other and composting) and it’s fun. You also get to take silly photos.

Bridget and Bonnie – friends who compost together stay together

How can you tell when it’s getting hot? When starting out, get yourself a thermometer, that way you can learn what 60/65 degrees looks/feels like. Once you’re feeling comfortable with the whole process your eyes and fingers do the job well.

Sadly the camera couldn’t catch the compost steam we were admiring above, indicating that our compost pile was hot and ready to be turned.

Here’s a photo from the Fork and Hoe Collective composting – the early morning light captured the compost steam beautifully.

We don’t cover our piles with any tarpaulins or carpet, we do put a thick layer of straw covering the whole pile to prevent the outside from drying out. However if you live in really high rainfall areas you may need to cover them to make sure it doesn’t get too drenched. We also make free-standing piles with no infrastructure required at all. This means they’re easier to make and turn.

How often should you turn it?

You need to turn the pile to make sure it ‘cooks’ evenly. When you actually turn it is determined by how quickly/slowly it reaches the desired temperature (60/65 degrees), this often happens within 24 hours, other times it can take up to 7 days days – it depends on the inputs. The books will generally say once a week, however the more obsessed you get with compost the more you will refine your practices.

Compost Activators

To help compost be the best it can be you can add compost activators – their job is to inoculate the compost pile with their nutrients, giving it a serious BOOST. Some of the common activators include some plants (comfrey leaves, dandelion, stinging nettle, yarrow leaves, tansy leaves) which all have especially high levels of key minerals. You can also use mature (healthy) compost which will be loaded with biology or road kill which will attract biology quick smart.

What does compost look like when it’s ready?

It’s the colour of 70% dark chocolate

It’s fluffy and has good ‘crumb’ structure, it doesn’t feel sticky/muddy or dry and sandy – it’s just right

When you squeeze a handful of it in your fist, one drop of water (no more) will come out of it – this indicates it has the right moisture content.

Swamped with small black flies: Make sure you have no exposed food scraps (cover with carbon).

Home to rats and mice: Reduce the amount of bread and meat and if using a small compost bin (which isn’t hot composting) add vermin mesh on the bottom of the compost bin to prevent rodents digging under.

Vermin mesh is sometimes used for bird aviaries, has really tough wire and very small squares ensuring that baby mice/rats can’t squeeze through. If you’re having trouble with rodents in your hot compost pile, turn it regularly and set some traps… If you’re into that kind of thing.

4 Responses to “How To Make HOT Compost”

i have been trying hot composting but something doesnt feel rite. its been over a month but the compost pile is still hot around 140F. The color has changed to blackish brown and smells sweet n earthy. I used sugar cane trash, rapseed trash and banana leaves as carbon source and for nitrogen i used food scraps and cow manure. C:N ration is around 30.1 and i always keep the pile covered with plastic. where am i going wrong? help would be appreciated.

Hi Ajay,
It actually sounds like things are going well for your compost pile in that it’s maintaining heat which is really useful for heating water (if you want to). In that context, 140F (approx 60degrees in my language) is the desired temperature to be reached – no more than this. I’m assuming your pile is quite large? As usually to maintain heat like this it needs to be quite large. If you’d like it to cool down, keep turning it semi-regularly and give it time – it will def get there. Let me know how you go.